Frequency-compensated coaxial attenuator having part of resistive film reduced and bridged by capacitance



United States Patent FREQUENCY-QOIVEPENSATED CGAXIAL ATTENU- ATOR HAVINGPART GE RESESTJEVE FiLD/i RE- DUCED AND BREGED BY APAC1TANCE Harry A.Norman, Seabrook, Md, assignor to Weinschel Engineering Co., Inc,Kensington, Md, a corporation of Delaware Filed Au 31, 1961, Ser. No.135,131 1 Claim. (Cl. 333- 51) This invention relates to a lossy-linetype of coaxial attenuator such as is used in high-frequency andmicrowave coaxial line circuits, and has for its primary object theprovision of a lossy-line type of attenuator which has a constantattenuation over a much wider frequency range than is possible withpresently-used types of attenuators. The usual lossy-line attenuator isuseful over a frequency range that is limited at the high frequency endby problems due to the coaxial connectors which are necessarily used, orby higher modes generated in the lines, that is, by factors outside theattenuator itself. At the low frequency end, the attenuatorcharacteristic tends to fall off rather rapidly below a certainfrequency, depending on the type and size of coaxial cable used, etc.One reason for this is that the attenuator must have a certain minimumlength, expressed in terms of wavelengths, to function properly as aneffective coaxial attenuator. If the resistive section is short comparedto a wavelength, it is apparent that the attenuation will not beconstant with respect to frequency; the major purpose of the presentinvention is to provide wide-band attenuation having substantially thesame value of attenuation, expressed in decibels, for a wide band offrequencies.

A major object of the invention is to provide a lossyline coaxialattenuator of the type using a very thin layer of resistive material asone of the elements of a section of coaxial line, with a seriescondenser which is structurally a part of the lossyline and whichfunctions in a novel way to offset the usual droop at the low end of theinsertion loss vs. frequency curve.

A further object is to provide such a construction without appreciablychanging the bull: or physical configuration of the attenuator, and bymeans of a simple construction which adds little to the expense offabrication or to the skill required to make the attenuator uni-t.

Another object is to provide a wide-band high-frequency coaxialattenuator of simple and rugged construction and having high stabilityand resistance to physical and thermal stresses and aging.

The invention is particularly applicable to attenuators of the typeshown in US. Patent to Weber, No. 2,689,294, for Metal Film Attenuator,and also to attenuators of the type shown in US. Patent to Weinschel,No. 3,002,166, for Inside-Out Attenuator for High-Frequency CoaxialLines, wherein the resistive coating is applied to the outer coaxialelement of a coaxial section instead of to the inner coaxial element.

The present invention is an improvement over the invention described inthe copending application of Weinschel et al., Serial No. 24,998, filedApril 27, 1960, for Frequency-Compensated Coaxial Attenuator, now PatentNo. 3,005,967. According to that invention, a circumferential gap ismade in the thin resistive tubular coaxial element intermediate the endsthereof, and a short section of the element in the vicinity of this gapis coated with a thin layer of insulating material over which is placeda further coating of conducting material which extends across the gapaxially for a suitable short distance on either side of the gap,forming, in effect, a series condenser arrangement, one plate of whichis the conducting outer layer, the other plate (or plates) 3', l 05,2 ll Patented Sept. 24, 1963 'ice being formed by the portions of theresistive layer under the coating of the conductive material. Thisfunctions partly as a series condenser and partly as a constructionwhich shorts out a portion of the resistive material at the higherfrequencies and introduces more of the resistive material at the lowerfrequencies to level out the attenuation-frequency characteristic aswill be explained in detail below.

According to the present invention, the circumferential gap is used, asbefore, but it is not out completely around the circumference of theresistive layer, leaving a continuous D.-C. conducting path between theterminals of the attenuator, which is in parallel with the condenserarrangement formed by the gap, as described in detail below. Thisprovides a greater range in variation of the overall impedance, bettercontrol of the frequency characteristic curve of the attenuator, andeasier and more accurate adjustment of small changes in value of theattenuator.

The specific nature of the invention, as well as other objects andadvantages thereof, will clearly appear from a description of apreferred embodiment as shown in the accompanying drawings, in which:

FIG. 1 is a longitudinal cross-sectional view of one form of attenuatoraccording to the invention;

FIG. 2, 3, and 4 show the central resistive element of the attenuator ofFIG. 1, in successive stages of preparation required to make thefinished product, and FIG. 5 is a graph used in explaining theinvention.

The invention will be explained in connection with an attenuator inwhich the inner coaxial element is the resistive element, similar tothat shown in the Weber patent previously referred to, but it will beunderstood that the same principle and construction are applicable to anattenuator of the type shown in the patent to Bruno O. Weinschel, No.3,002,166, wherein the resistive coating is applied to the outer coaxialelement.

FIG. 1 is a longitudinal sectional view of an attenuator made accordingto the invention, in the usual form of an element which can be inserteddirectly into a coaxial line, and for this purpose is provided withstandard coaxial male and female fittings 2 and 3 respectively. Theouter coaxial conductor of the attenuator, 4, is of conventionalconstruction. In this case, the actual attenuator element is the innerconductor 6, which is essentially a rod of insulating material, usuallyof ceramic material, provided with bullet connectors 7 and 8 at the endsthereof mating with the central terminal elements of the respectivecoaxial fittings 2 and 3. In a prior type of construction, such as shownin the copendin g application above referred to, the resistive elementis formed of a very thin coating 9, which may be either an alloy ofnoble metals, or a thin layer of carbon suitably applied, by previouslyknown methods and techniques which are not a part of the presentinvention. In the usual attenuator construction, this coating uniformlycovers the ceramic rod 6 from end to end, and constitutes the resistiveattenuator element. In this usual, construction, however, the attenuatorelernent typically has the characteristic that while at, for example, 3db the insertion loss is reasonably constant over range from l-12.4kilomegacycles, at 10 db, the insertion loss falls oif very badlybetween 1 and 4 krnc. In making a 10 db attenuator, it is necessary touse a much higher resistance rod, and the ratio of resistance towavelength goes up quite rapidly, and the attenuation correspondinglygoes down for the lower frequencies, as explained above. This can befurther understood by considering that if we assume a lossless line, thecharactenistic impedance is determined by the distributed inductance andcapacitance of the line only, but in the case of an attenuator, it isnecessary to make one of the conductors resistive, whereupon thecharacteristic impedance becomes frequency sensitive if the ratio ofresistance to wavelength becomes too high, as it does for the lowerfrequencies. This will be apparent from consideration of the standardtransmission line equations, which show that all of the parameters areexpressed in terms of quantities which can be transformed into a ratioof wavelengths to the physical parameters of the line.

In order to minimize the drooping characteristic at the lower frequencyrange, the construction shown in the drawings is employed. For thispurpose, the central conductor is prepared in the usual fashion, thatis, the ceramic rod 6 is coated with the usual resistive layer 9 fromend to end. A nan'ow circumferential groove, typically 30-60 mils wide,is then cut in the resistive film, for a predetermined portion of thecircumference, say 180". This can be conveniently done by grinding, thepurpose being to remove the resistive film, but as little of the ceramicas possible. The resulting construction is as shown in FIG. 2. A coat ofinsulating material, for example, black Glyptai, is now applied to thevicinity of the gap 11, as shown at 12. This may be done with a smallcamel-hair brush, or by any other convient means, and should extend, ina typical case, at least one-fifth inch on either side of the groove 11,as well as covering the groove. If Glyptal is used, the rod should thenbe baked at 150 C. fora minirnum of eight hours, then a second coat ofGlyptal should be applied and baked at the same temperature "for anadditional eight'hours. A band of silver paint, 13, should now beapplied directly over the groove, but not extending as far as theGlyptal, and baked for a minimum of four hours at 150 C. to provide, ineffect, a tubular coating of conductive material over this portion ofthe rod. It will be understood that instead of silver, any othersuitable conductive material may be similarly applied, and that band 13may be of resistive material instead of highly conductive, depending onthe characteristic required.

It will be apparent that the attenuator now has a continuous resistiveportion spanning the gap at 10, and a capacitive portion in parallelwith this resistive portion. By varying the circumferential distance ofthe gap, the resistance of path It? can be adjusted, while by varyingeither the circumferential length of the silver strip 13, or its axiallength, the capacitive portion can be adjusted.

In order to determine the proper axial length of the conductive coating13, the attenuator may then be assembled, and the attenuation measuredat both the high and low ends of the frequency spectrum desired, say forexample, from 0.5 kmc. to 11.0 kmc. The attenuation should not differ bymore than +0.1 db to -02 db at these two frequencies. If the attenuationis greater than this, the axial length f 13 should be increased, andvice versa. This can readily be done by either adding a little silverpaint to extend the conductive coating, or else by scratching oil alittle of the paint to reduce the area of the coating.

As a final production step, the entire rod unit 6 may then be given anadditional coat of black Glyptal from end to end, and baked again for aminimum of eight hours at 150 C. As indicated by the above procedure, itis normally expected that an attenuator so made will have asubstantially flat frequency-attenuation characteristic, within thelimits noted, over the entire frequency band from 0.5 to at least 11kmc. This is in sharp contrast to the characteristic of an ordinaryattenuator as noted above.

It will be noted that the improved result is not due simply to theeffect expected by placing a capacitor in series with a resistive line,since a pure capacitive element is not dissipative and does not producethe same effect as a lossy-line resistance type of attenuator. Alossy-line attenuator is an attenuator in the form of a line sectionwhich is deliberately made uniformly resistive along its length so thatthe attenuation is a function of i 1 the length of the line section. inthe above construction, the groove 11 provides, together with theadjacent ends of the resistance material 9 and the conductive layer 13,a structure corresponding to two capacitors in series and a resistor inpanallel with them. However, this is obviously not a lumped capacity,but the capacity is also distributed along a portion of the lineadjacent the groove. Due to this construction, a portion of the currentin the resistive film at very high frequencies will mostly be bypassedright into the silver from the resistive films, tending to :go directlyfrom the resistive film 9 to one end of the conductive band 13, and thento return into the resistive film at or very near the other end. In thiscase, only a small part of the capacity will be effective; furthermore,since there is a larger eflective length of the outer silver coatingacting in this case, there is correspondingly a shorter effective lengthof the resistive element being utilized. Conversely, at the lowerfrequencies more and more of the inner surface of the silver coating 13is effective as an ordinary condenser, and more and more of theresistive material'enters into the action. Therefore, at the lowerfrequencies, the attenuation does not fall oti, but tends to remainsubstantially uniform throughout the entire range. It will be apparentfrom the foregoing that the distribution on? current along the length ofthe capacitor-resistor section is different at different frequencies ina manner which tends to compensate for the usual low-frequency droop.

While exact calculation of the effect at each frequency is difficult, aspreviously noted, approximate calculations can be easily made withsimplifying assumptions, which give sufiiciently good results so that itis not difficult to compute the approximate dimensions for any given setof practical conditions, using as a basis the capacitive reactance of acondenser of the dimensions which are determined by the size of coaxialcable and other parameters employed.

The above construction is'of particular utility where a flatcharacteristic is essential, but adds so little to the cost of theattenuator, that it may be used in any situation where a general-purposewide spectrum attenuator is required. It does not adversely affect theoperation of the attenuator in any way, and the stability, aging, shockre sistance, etc., are fully equal to those of an ordinary attenuator ofthis general type. The units are easily reproducible as to electricalcharacteristics in manufacture by the use of ordinary production controlmethods, and do not require more highly skilled labor than the ordinaryattenuators.

While the band 13 has been shown as of conductive material, it will beunderstood that this could also be of resistive material, which may bedesirable in some cases to produce a particular characteristic.

In FIG. 2, the gap 11 has been shown as being further from one end ofthe rod 6 than from the other. This is the preferred construction in thecase of a unilateral attenuator, that is, one which is always to be fedfrom one end. In this case, it is desirable to put the discontinuityfairly close to the other end, so as to provide the maximum attenuationpath for any slight reflections which may occur due to the unavoidableeffects of the discontinuity. In the case of a bilateral attenuator,that is, one which is intended for use in both directions oftransmission, then the gap 11 should obviously be placed at the centerof the rod 6.

In FIG. 5, the curve A shows the normal increase in attenuation withfrequency; curve B shows the improvement using a gap cut all the wayaround the resistance film 9, as shown in Weinschel Patent No.3,005,967. It will the noted that this improves the characteristiccurve, but tends to introduce a hump at B. Curve C shows the furtherimprovement introduced according to the present invention, by leavingsome resistance film across the ends of the gap. In addition to thisimprovement, the VSWR is also improved, especially for low attenuationvalues,

e.g., 3-6 do or so. Inclusion of the capacitor alone tends to produce aless favorable VSWR, especially at the lows frequency end of thecharacteristic curve. Providing the resistive film portion across thegap tends to reduce the VSWR, and by careful design an optimumarrangement can be found which greatly improves the flatness of thecharacteristic, and thereby extends the efiective range Off theattenuator, without too adversely effecting the VSWR.

it will be apparent that the embodiments shown are only exemplary andthat various modifications can he made in construction and arrangementwithin the scope of my invention as defined in the appended claim. Forexample, the resistive coating could be on the inner surface of theouter coaxial conductor, as in the Weinschel patent previously referredto, in which case the circumferential slit would be made as before, buton the inside surface of a hollow cylinder, and the respective bands ofinsulation and silver would be respectively nearer the central axis thanthe main resistive coating instead of being further away from thecentral axis, but otherwise the construction and principle of operationwould be the same.

I claim:

A high-frequency coaxial attenuator having opposed conductivecylindrical surfaces lying between coaxial terminals, one of saidsurfaces being constituted by a thin layer of resistive material adheredto the surface of an insulating member having a cylindrical-outersurface to constitute a lossy-attenuator surface, and extendingcontinuously along said surface between said coaxial terminals exceptfor a narrow gap extending part of the Way around the circumference ofsaid surface; condenser means including the resistive material of saidlayer adjacent said gap to provide with said gap 21 frequency-sensitivecomplex impedance which has a relatively low impedance value at highfrequencies and a relatively higher impedance value at low frequencies,said impedance being effectively in parallel with the portion of theresistive layer which is adjacent the ends of said gap, whereby thefrequency-attenuation characteristic of the attenuator is made moreuniform over an extended range.

References Cited in the file of this patent UNITED STATES PATENTS2,030,178 Potter Feb. 11, 1936 2,524,857 Seeker Oct. 10, 1950 2,667,622Weber et a1. J an. 26, 1954 2,686,295 Griesman Aug. 10, 1954 3,005,967Weinschel Oct. 24, 1961

